My Monte has almost 150,000 mi. on it. At 135,000 it had a full transmission service done, as indicated. In the last 2 months I have noticed it clunking when shifting from 1st to 2nd, then kind of slipping then grabbing when going from 2nd to 3rd. It mostly happens in the city when stopping and going a lot and in colder weather. It has started to become more frequent here lately. I have a sealed transmission so no fluids are ever added, and it is a 6 cylinder. Is my transmission failing?